1. Field of the Invention
The invention relates generally to the transmission of digital data along a communications bus and, more particularly, to serial communications over a communications bus containing a minimum number of conductors.
2. Discussion of the Related Art
Often, communications between two circuits take place over a communications bus between a "master" circuit and a "slave" circuit. In such an arrangement, the slave circuit typically sends data along the bus only in response to a request from the master circuit. This is particularly the case where a master circuit needs to communicate with a number of interchangeable or removable slave circuits. The slave circuits may include memory devices for storing information.
Such slave circuits may be, for example, tags used to identify articles, which are either brought only momentarily into contact with the communications bus, or which need to provide some information about themselves the first time that they are brought into contact with this bus.
Generally, a communications bus for use between a microprocessor master circuit and a memory device slave circuit comprises at least four conductors. A ground voltage supply, a power supply voltage, a clock signal and a bidirectional data conductor. Where chip select and deselect commands are needed, such conventional buses may include a dedicated combination of clock and data signals which are interpreted as chip select/deselect commands by the slave circuits.
Conventional communications buses are generally implemented as a two-wire communications bus. A typical two-wire communications bus is used in the Dallas Semiconductors DS2400 product. The communications bus includes one ground conductor and one signal conductor. The signals which are transmitted along the signal conductor typically comprise pulses of 0V to 5V relative to the ground conductor. A drawback to this known two conductor bus is that it does not efficiently transmit the power required by the slave circuit. Another disadvantage is that the master and slave circuits are not synchronized, as no clocking signals are common to both the master and slave circuits.
It is therefore an object of the invention to provide a communications bus which can communicate on as few a number of conductors as possible. This will allow simple installation of the bus, and allow the slave circuits to be packaged in very small, cheap packages.